The present invention relates to a false contour correcting apparatus and method used when an image signal is subjected to digital signal processing.
In recent years, as a television receiver or the like has increased in image quality and made multi-functional, an image signal has been frequently subjected to digital signal processing. For example, gray level correction has been made by the digital signal processing in order to increase the contrast of an image. When a quantized digital signal is subjected to such digital signal processing, an unnatural contour referred to as a false contour may, in some cases, be produced in an image displayed on a screen. To address this, a technique for removing a false contour produced by digital signal processing is disclosed in JP-A-6-62280, for example.
Referring now to the drawings, description is made of a conventional false contour correction circuit which is disclosed in JP-A-6-62280. FIG. 9 is a block diagram showing the configuration of the conventional false contour correction circuit. The false contour correction circuit comprises a random number generator 5, a judgment circuit 6, and an addition circuit 7, and receives an n-bit digital image signal A. A signal F composed of predetermined lower bits out of the n bits composing the inputted digital image signal A is fed to the judgment circuit 6. The random number generator 5 outputs a digital random number H having the same bit width as the bit width of the signal F. The judgment circuit 6 compares a value represented by the signal F composed of the predetermined lower bits of the digital image signal A with the digital random number H outputted from the random number generator 5, and outputs a signal representing xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d as a correction signal I depending on the results of the comparison. The addition circuit 7 is an adder having the same bit width as that of upper bits G of the digital image signal A, and adds the upper bits G of the digital image signal A and the correction signal I outputted from the judgment circuit 6, to generate a corrected output signal J.
According to the above-mentioned false contour correction circuit, the correction signal I having no regularity is added to the upper bits G of the digital image signal A. Therefore, the signal F composed of the lower bits which are inputted to the judgment circuit 6 out of the n bits composing the digital image signal A and the correction signal I outputted from the judgment circuit 6 are not correlated with each other within the precision of the random number generator 5. In a case where an image which hardly changes in brightness or hue is quantized by thus making a correction having no correlation with the image, the position on a screen where the quantization level changes is dispersed backward, forward, right, and left, so that an unnatural false contour is reduced. Consequently, such a digital image signal after the correction makes it possible to obtain an image whose image quality is prevented from being lowered by quantization whose level is low.
In the conventional false contour correction circuit, the digital image signal is subjected to a variation corresponding to a change in the most significant bit without being correlated with an image so that the position on the screen where the quantization level changes is not correlated with the image signal. Therefore, even when a signal representing an image whose brightness is constant is inputted, an image which contains noises roughing the display on the screen is obtained by the variation of the image signal corresponding to the change in the most significant bit.
Therefore, an object of the present invention is to provide a false contour correcting apparatus capable of reducing a false contour in an image based on a digital image signal while avoiding lowering the image quality by such a side effect or the like caused by false contour correction in which the above-mentioned noises occur.
A first aspect of the present invention is directed to a false contour correcting apparatus for reducing a false contour in an image based on a digital image signal, comprising:
a double bit change detection circuit for detecting in the digital image signal a signal value change which is twice the minimum quantization unit of the digital image signal as a double bit change and outputting a signal representing the results of the detection as a double bit change detection signal; and
a signal correction circuit for subjecting a double bit change portion, which is a portion where the double bit change exists, in the digital image signal to correction for reducing the false contour on the basis of the double bit change detection signal.
According to the first aspect of the present invention, the double bit change in the digital image signal is detected, and the double bit change portion in the digital image signal is subjected to the correction for reducing the false contour. Therefore, it is possible to reduce the false contour while avoiding lowering the image quality by such a side effect or the like in conventional false contour correction where noises occur.
According to a second aspect of the present invention, in the first aspect of the present invention,
the signal correction circuit corrects the double bit change portion in the digital image signal into a portion where there exist two one-bit changes each of which is a signal value change corresponding to the minimum quantization unit on the basis of the double bit change detection signal.
According to the second aspect of the present invention, the double bit change in the digital image signal is converted into the two one-bit changes, so that the false contour corresponding to the double bit change is removed or reduced.
According to a third aspect of the present invention, the false contour correcting apparatus in the first aspect of the present invention further comprises
a front and rear flatness detection circuit for judging whether or not there exists a signal value change which is not less than twice the minimum quantization unit in predetermined sections before and after the double bit change in the digital image signal, and outputting a signal representing the results of the judgment as a flatness detection signal,
the signal correction circuit subjecting, only when there exists no signal value change which is not less than twice the minimum quantization unit in the predetermined sections before and after the double bit change, the double bit change portion to the correction for reducing the false contour on the basis of the flatness detection signal.
According to the third aspect of the present invention, only when there is no change which is not less than the double bit change in the predetermined sections before and after the double bit change in the digital image signal, that is, only when the value of the digital image signal is approximately constant in the predetermined sections, the double bit change is corrected. Therefore, it is possible to reliably correct only the false contour, and prevent an image from being adversely affected by the correction for the false contour.
According to a fourth aspect of the present invention, in the third aspect of the present invention, the predetermined sections are sections respectively corresponding to five adjacent pixels in the digital image signal.
According to a fifth aspect of the present invention, the false contour correcting apparatus in the second aspect of the present invention further comprises
a front and rear flatness detection circuit for judging whether or not there exists a signal value change which is not less than twice the minimum quantization unit in predetermined sections before and after the double bit change in the digital image signal, and outputting a signal representing the results of the judgment as a flatness detection signal,
the signal correction circuit subjecting, only when there exists no signal value change which is not less than twice the minimum quantization unit in the predetermined sections, the double bit change portion to the correction for reducing the false contour on the basis of the flatness detection signal, and determining the positions of the two one-bit changes such that an interval between the one-bit changes is shorter than the predetermined sections in the correction.
According to the fifth aspect of the present invention, only when there exists no other double bit change in the predetermined sections before and after the double bit change in the digital image signal, the correction is made. The interval between the two one-bit changes produced by the correction is shorter than the predetermined sections. Consequently, correction for one of the two double bit changes existing in the digital image signal does not adversely affect correction for the other double bit change. Therefore, it is possible to also perform false contour correction processing having no erroneous operation with respect to the digital image signal in which the double bit change frequently occurs.
According to a sixth aspect of the present invention, the false contour correcting apparatus in the second aspect of the present invention further comprises
a random number generation circuit for generating a random number signal representing a pseudorandom number,
the signal correction circuit determining, on the basis of the random number signal, the positions of the two one-bit changes to be produced by the correction for the double bit change portion.
According to the sixth aspect of the present invention, the positions of the two one-bit changes produced by the correction for the double bit change portion in the digital image signal are determined at random on the basis of the random number signal. Therefore, it is possible to prevent such a phenomenon that portions where the one-bit changes respectively exist (hereinafter referred to as one-bit change portions) look like a vertical line upon being longitudinally lined up in an image based on the digital image signal.
According to a seventh aspect of the present invention, in the sixth aspect of the present invention,
the random number generation circuit receives a horizontal synchronizing signal corresponding to the digital image signal, and outputs as the random number signal a signal representing a value which varies depending on a horizontal line represented by the horizontal synchronizing signal.
According to the seventh aspect of the present invention, the positions of the two one-bit changes produced by the correction for the double bit change portion in the digital image signal are irregularly shifted right and left for each horizontal line on the basis of the random number signal. Therefore, it is possible to prevent such a phenomenon that the two one-bit change portions look like a vertical line upon being longitudinally lined up in an image based on the digital image signal.
An eighth aspect of the present invention is directed to a false contour correcting apparatus for reducing a false contour in an image based on a digital image signal, comprising:
a change detection circuit for detecting in the digital image signal a signal value change which is not less than twice the minimum quantization unit of the digital image signal, and outputting a signal representing the results of the detection as a change detection signal;
a front and rear flatness detection circuit for judging whether or not there exists a signal value change which is not less than twice the minimum quantization unit in predetermined sections before and after the signal value change detected by the change detection circuit in the digital image signal, and outputting a signal representing the results of the judgment as a flatness detection signal; and
a signal correction circuit for subjecting, only when there exists no signal value change which is not less than twice the minimum quantization unit in the predetermined sections, a portion where there exists the signal value change detected by the change detection circuit in the digital image signal to the correction for reducing the false contour on the basis of the change detection signal and the flatness detection signal.
According to the eighth aspect of the present invention, only when the change which is not less than the double bit change in the digital image signal is detected, and the value of the digital image signal is approximately constant in the predetermined sections before and after the double bit change, the portion where there exists the change which is not less than the double bit change in the digital image signal is subjected to the correction for reducing the false contour. Therefore, eve when a change which is not less than three times the minimum quantization unit exists in the digital image signal, and a false contour is produced by the change, the false contour can be reduced.
A ninth aspect of the present invention is directed to a false contour correcting method for reducing a false contour in an image based on a digital image signal, comprising:
a double bit change detecting step of detecting, as a double bit change portion, a portion where the values of adjacent pixels differ by a value which is twice the minimum quantization unit of the digital image signal in an image represented by the digital image signal; and
a correcting step of correcting the values of the pixels in the double bit change portion, to reduce the false contour.
According to a tenth aspect of the present invention, in the ninth aspect of the present invention,
in the signal correcting step, the values of the pixels in the double bit change portion are corrected on the basis of the results of the detection in the detecting step such that there exist two one-bit change portions, each of the one-bit change portions being defined as a portion where one-bit change exists and the values of the adjacent pixels differ by the minimum quantization unit.
According to an eleventh aspect of the present invention, the false contour correcting method in the ninth aspect of the present invention further comprises
a front and rear flatness detecting step of detecting, in the predetermined sections before and after the adjacent pixels in the double bit change portion, whether or not there exist adjacent pixels whose values differ by not less than twice the minimum quantization unit,
in the correcting step, the values of the pixels in the double bit change portion being corrected, only when no adjacent pixels whose values differ by not less than twice the minimum quantization unit exits in the predetermined sections, on the basis of the results of the detection in the front and rear flatness detecting step.
According to a twelfth aspect of the present invention, the false contour correcting method in the tenth aspect of the present invention further comprises
a front and rear flatness detecting step of detecting, in the predetermined sections before and after the adjacent pixels in the double bit change portion, whether or not there exist adjacent pixels whose values differ by not less than twice the minimum quantization unit,
in the correcting step, the values of the pixels in the double bit change portion being corrected, only when no signal value change which is not less than twice the minimum quantization unit exists in the predetermined sections, on the basis of the results of the detection in the front and rear flatness detecting step, and the positions of the two one-bit change portions being determined such that an interval between the one-bit change portions is shorter than the predetermined sections in the correction.
According to a thirteenth aspect of the present invention, the false contour correcting method in the tenth aspect of the present invention further comprises
a random number generating step of generating a pseudorandom number,
in the correcting step, the positions of the two one-bit change portions to be produced by the correction for the double bit change portion being determined on the basis of the pseudorandom number.
A fourteenth aspect of the present invention is directed to a false contour correcting method for reducing a false contour in an image based on a digital image signal, comprising:
a change detecting step of detecting, in an image represented by the digital image signal, a change portion where the values of adjacent pixels differ by not less than twice the minimum quantization unit of the digital image signal;
a front and rear flatness detecting step of detecting, in predetermined sections before and after the adjacent pixels in the change portion detected in the detecting step, whether or not there exist adjacent pixels whose values differ by not less than twice the minimum quantization unit; and
a correcting step of correcting the values of the pixels in the change portion detected in the change detecting step, only when no adjacent pixels whose values differ by not less than twice the minimum quantization unit exist in the predetermined sections, on the basis of the results of the detection in the change detecting step and the results of the detection in the front and rear flatness detecting step, to reduce the false contour.